I was an ER nurse in 4 countries for 40 years. My heart sinks when I see people donning masks that were stuffed in their pockets or swinging from the rear view mirror of their vehicle. If you feel the need to mask ( do not advise it because it as no effect on the spread of respiratory infections and may facilitate them) at least use a clean fresh one supplied by the stores and discard it when you leave the store. While masks do not filter viruses or some bacteria, masks do trap spores, fungi and dust. With your warm breath you have an invisible vegetable garden of bacteria and fugus growing right in front of your airway. Use a clean mask in every store and discard it at the door, at least that way you reduce the risk of bacterial or fungal pneumonia ( far more serious than viral pneumonia).